1. Field of the Technology
The present disclosure relates to methods of forging metal alloys, including metal alloys that are difficult to forge due to low ductility. Certain methods according to the present disclosure impart strain in a way that maximizes the buildup of disorientation into the metal grain crystal structure and/or second-phase particles, while minimizing the risk of initiation and propagation of cracks in the material being forged. Certain methods according to the present disclosure are expected to affect microstructure refinement in the metal alloys.
2. Description of the Background of the Technology
Ductility is an inherent property of any given metallic material (i.e., metals and metal alloys). During a forging process, the ductility of a metallic material is modulated by the forging temperature and the microstructure of the metallic material. When ductility is low, for example, because the metallic material has inherently low ductility, or a low forging temperature must be used, or a ductile microstructure has not yet been generated in the metallic material, it is usual practice to reduce that amount of reduction during each forge iteration. For example, instead of forging a 22 inch octagonal workpiece to a 20 inch octagon directly, a person ordinarily skilled in the art may consider initially forging to a 21 inch octagon with forging passes on each face of the octagon, reheating the workpiece, and forging to a 20 inch octagon with forging passes on each face of the octagon. This approach, however, may not be suitable if the metal exhibits strain-path sensitivity and a specific final microstructure is to be obtained in the product. Strain-path sensitivity can be observed when a critical amount of strain must be imparted at given steps to trigger grain refinement mechanisms. Microstructure refinement may not be realized by a forge practice in which the reductions taken during draws are too light.
In a situation where the metallic material is low temperature sensitive and is prone to cracking at low temperatures, the on-die time must be reduced. A method to accomplish this, for example, is to forge a 22 inch octagonal billet to a 20 inch round cornered square billet (RCS) using only half of the passes that would be required to forge a 20 inch octagonal billet. The 20 inch RCS billet may then be reheated and the second half of passes applied to form a 20 inch octagonal billet. Another solution for forging low temperature sensitive metallic materials is to forge one end of the workpiece first, reheat the workpiece, and then forge the other end of the workpiece.
In dual phase microstructures, microstructure refinement starts with sub-boundary generation and disorientation buildup as a precursor to processes such as, for example, nucleation, recrystallization, and/or second phase globularization. An example of an alloy that requires disorientation build up for refinement of microstructure is Ti-6Al-4V alloy (UNS R56400) forged in the alpha-beta phase field. In such alloys, forging is more efficient in terms of microstructure refinement when a large reduction is imparted in a given direction before the workpiece is rotated. This can be done on a laboratory scale using multi-axis forging (MAF). MAF performed on small pieces (a few inches per side) in (near-) isothermal conditions and using very low strain rates with proper lubrication is able to impart strain rather homogeneously; but departure from any of these conditions (small scale, near-isothermal, with lubrication) may result in heterogeneous strain imparted preferentially to the center as well as ductility issues with cold surface cracking. An MAF process for use in industrial scale grain refinement of titanium alloys is disclosed in U.S. Patent Publication No. 2012/0060981 A1, which is incorporated by reference herein in its entirety.
It would be desirable to develop a method of working that provides sufficient strain to a metallic material to initiate microstructure refinement mechanisms efficiently through forging, while limiting ductility issues.